As weird as this is going to sound, check your coolant reservoir for your turbo and radiator. If they are low or empty, top them off with 50/50 coolant. From my understanding when the coolant is low, the DAM of your car will drop below the value of 1.0 and therefore reducing the target boost level. Then reset your ECU with Cobb AP

That is what I was told to do when I had the same problem and it worked for me. Once I datalogged the car it all made sense. Datalog it and post up the file and it will narrow down some possibilities. I'd also perform a compression test to be on the safe side -$30 kit from AutoZone, about 2-3 hours to do it yourself with some instructions from NASIOC/IWSTI wouldn't hurt.